According to atmospheric turbulence phase structure function, the relation between the angle-of-arrival covariance of two points and the atmospheric turbulence outer scale is deduced, and normalized by the differential angle-of-arrival variance. The expression of outer scale by the ratio of angle-of-arrival variance and angle-of-arrival covariance is obtained. The experimental results for horizontal propagation in the real atmosphere by four-aperture angle-of-arrival variance measuring apparatus show that the average outer scale is about 4 m at the height of 6 m above the ground. Further more the outer scale changes with time, and they are different in different directions.